This invention relates generally to an electrophotographic printing machine having a seamed, web-type photoreceptor suitable for the exposure of one or more document images on the surface thereof, and more particularly to a method and apparatus for controlling the location of the photoreceptor seam in relation to the document images.
The features of the present invention may be used in the printing arts and, more particularly in electrophotographic printing. In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is then image-wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a copy sheet and permanently affixed thereto. The foregoing description generally describes a typical single color electrophotographic copying machine.
A typical machine of this type would be the Xerox.RTM. 1090.RTM. copier. Such a machine employs a mechanical rephaser to control the position of the photoreceptor seam with respect to the exposed or latent image areas of the photoreceptor. Generally, the rephaser is a gear box having two speeds for control of the speed at which the copy sheet is advanced as it is brought into registration with the latent image on the photoreceptor. Generally, the copy sheet transport system is used to trigger the exposure mechanism which creates the latent image on the photoreceptor. Periodically, once per photoreceptor revolution, the location of the photoreceptor seam is sensed and the rephaser is energized or deenergized for a period of time necessary to correct for the positioning of the advancing copy sheet, and in turn, the position of the latent image on the photoreceptor web. More specifically, the photoreceptor belt is moved at a predefined velocity, and the rate of travel of the advancing copy sheet is controlled so as to regulate the exposure and transfer operations in accordance with the position of the advancing sheet. Minor variations in the speed of the main drive motor, due to variations in the power line voltage, result in a variation of the position of latent images on the photoreceptor. Unfortunately, these variations are cumulative in nature and must be corrected to assure that the latent images are exposed at generally the same position on the photoreceptor each time. If not corrected, the cumulative variation would eventually cause one of the exposed latent image areas to occur over the photoreceptor seam, subsequently resulting in an unacceptable copy.
Such a system works well for typical single color systems, such as the Xerox.RTM. 1090.RTM. copier, but lacks the reliability for accurate velocity and position control of the photoreceptor required in multicolor development systems. Also, after significant variations have occurred in the photoreceptor velocity, resulting in the mis-positioning of the photoreceptor seam, the system may require a "dead" or nonoperative cycle, during which the copier once again repositions the seam to the interdocument region. Furthermore, the rephaser mechanism is a relatively expensive apparatus which provides the mechanical drive linkage between the photoreceptor drive and the copy sheet transport system. Hence, a more flexible and less costly drive system would be desirable.
Another technique used to control two moving members in a reprographic system is illustrated by U.S. Pat. No. 3,917,400 to Rodek et al. (Issued Nov. 4, 1975) which discloses a method and apparatus for maintaining a predetermined phase relationship between signals representing the velocity of a first variable velocity movable member and the velocity of a second constant velocity movable member. A first sensor emits a pulse signal whenever one of a plurality of registration marks on the variable velocity movable member passes the sensor, and similarly, a second sensor emits a pulse whenever one of a plurality of registration marks on the constant velocity movable member passes the second sensor. A phase relationship between the two movable members is determined by measuring the phase relationship between the occurrence of the pulse signal of the first sensor and the pulse signal of the second sensor. A control signal, related to the phase relationship, is generated and is utilized to vary the velocity of the variable velocity movable member so that a predetermined phase relationship (i.e. zero phase difference) is established for the two signals. Furthermore, a portion of the control signal generated to reduce the signal to zero is used to reduce a subsequent phase difference calculation to zero, thereby compensating for the fact that the velocity of the variable velocity movable member is still being adjusted as the subsequent difference calculation is being made.
A related method of positioning an electrostatic latent image on a photoconductive belt is described in U.S. Pat. No. 4,980,723 to Buddendeck et al. (Issued Dec. 25, 1990), and is hereby incorporated by reference for the teachings therein. The reference discloses a system capable of adjusting the number of latent image regions which are exposed on the photoconductive belt. More specifically, a portion of the inter-image zone is utilized to accomodate the shifting of the latent image positions on the belt. Furthermore, a control system for automatically altering the pitch, or number of latent images on a photoconductive belt, during operation is taught by U.S. Pat. No. 4,588,284 to Federico et al. (Issued May 13, 1986), where a memory flag is monitored to control the selection of a different number of pitches. The flag is also used to control the clock signals used for the timed actuation of events with respect to the selected pitch. The relevant portions of U.S. Pat. No. 4,588,284 to Federico et al. are hereby incorporated by reference.
The present invention seeks to overcome the limitations of the mechanical rephaser type control system, by mechanically decoupling the photoreceptor drive from the copy sheet registration and transport drives. Moreover, the present system has the added advantage of being able to control the phase relationship between two independently variable elements, the photoreceptor speed and the advancing copy sheets, in a reliable manner.
In accordance with one aspect of the present invention, there is provided a method for controlling the velocity of the photoreceptor within a reprographic machine of the type having a seamed, web type photoreceptor, for producing a plurality of developed images thereon, said developed images being separated by unexposed interdocument regions or zones on the photoreceptor, and means for registering copy substrates with the developed images. The method of assuring that the seamed region of the photoreceptor lies within an interdocument region begins by first sensing an actual phase relationship between the photoreceptor seam and the activity of the sheet registration apparatus. The method then calculates a phase error value by comparing the actual phase relationship between the photoreceptor seam and the registration apparatus to a desired phase relationship. As the next step, the system determines a new photoreceptor speed as a function of the phase error. Finally, the photoreceptor is accelerated or decelerated to a new constant velocity during interdocument gaps. The new constant velocity remains in effect during the subsequent exposure of the latent images. During operation of the reprographic machine, the above steps are executed once per revolution of the photoreceptor.
Pursuant to another aspect of the present invention, there is provided an electrophotographic printing machine of the type having a seamed, web type photoreceptor, for producing a plurality of developed images thereon, where the developed images are separated by unexposed interdocument zones on the photoreceptor. The machine also has an independently driven copy substrate registering apparatus for registering copy sheets in synchronization with the developed images on the photoreceptor. Included in the machine are phase measurement means for quantizing the phase relationship between the photoreceptor seam and an edge of the advancing copy sheets, and phase error calculating means for determining the variation in the phase relationship with respect to a desired phase relationship. Also included is a controller for adjusting the photoreceptor speed as a function of the phase error, during the interdocument zones, and then driving the photoreceptor at a constant velocity during image exposure.